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A clip-on sensor the size of a fly's wing could save farmers' time and money, scientists say.

The sensor works with a wireless network to alert the farmer when crops need water or can even start irrigation systems automatically.

"We're talking about saving 30-40% of water used," says the technology's inventor, Dr Hans Seelig, a research associate at the University of Colorado at Boulder's BioServe Space Technologies Center.

"If you have an irrigation system that applies water in certain sectors, you could be even more efficient," he says.

The current prototype, which shows the sensor works, uses wires.

But Seelig and his team are working towards a wireless version that uses radio frequency identification, or RFID.

This technology allows not only the data, but also the power to be transmitted wirelessly, eliminating the need for batteries.

The sensor would be clipped permanently to a leaf during the growing season to monitor moisture content and chemical signatures that can indicate when the plant is undergoing water stress.

The chemical signs, such as an increase in salt and sugar content in the cells, occur much earlier than physical signs, such as drooping leaves, that many farmers rely on now.

Because it can only transmit a signal about half a metre away, the RFID tag can do one of two things with the data: it can transmit it to RFID tags nearby, which would then push the signal along to other tags in the network until the data reaches home base.

Alternatively, it can deliver the signal to a nearby base station, which would have enough power (using a battery or solar panel) to transmit the data directly to the farmer's computer.

Software on the computer would then analyse the data and could alert the farmer by email or text message. Or the computer could be set up to go one step further and automatically turn on the irrigation system.

Some farmers water their fields every day. But with the efficiency of a sensor system, they could instead water them every 30-40 hours, says Seelig, saving water.

"I think the time has come when we're all going to have to save water," says Greg Cure, a 31-year-old farmer who manages bean, corn, wheat, and potato fields in northeastern Colorado.

But for some farmers, saving water may not be the biggest incentive.

"The biggest incentive is saving labour," says Dr Robert Evans, research leader at the USDA Agricultural Research System Unit in Sidney, Montana.

If a new technology is going to be adapted, says Cure, a farmer would probably need some additional motivation such as helping with funding.

"There are guys out here in their 60's and 70's and they are going to do it the way they've always done it," he says.

Seelig has tested the technology on cowpeas, beans and sugar beet.

The University of Colorado has recently licensed the sensor commercially.